1. Field of the Invention
The present invention relates to a photosensitive resin composition, to a method of using the composition for patterning, and to electronic components made using the photosensitive resin composition.
2. Description of the Related Art
Recently, in the semiconductor industry, organic substances with good heat resistance such as polyimide resins and the like have been used as interlayer insulating materials, because of their good characteristics, in place of conventional inorganic materials. Circuit patterning of semiconductor integrated circuits and printed circuits requires many complicated steps of, for example, forming a resist film on the surface of a substrate, removing the unnecessary part of the film through selective exposure and etching at predetermined sites, and rinsing the surface of the thus-processed substrate. It is therefore desired to develop heat-resistant photosensitive materials for use as photoresists that can be directly used as insulating layers after having been patterned through exposure and development.
Heat-resistant photosensitive materials have been proposed, for example, comprising, as a base polymer, photosensitive polyimide, cyclic polybutadiene or the like. Above all, photosensitive polyimides are specifically noted, because their heat resistance is good and impurities (e.g., water, solvents, photosensitive groups of the polymer, photoinitiators, sensitizers, etc.) can be removed. Among such photosensitive polyimides, there has been proposed a system comprising a polyimide precursor and a bichromate (see JP-B 49-17374). The proposed system had the advantages of useful light sensitivity and good film forming ability, but is defective in that its storage stability is poor and that chromium ions remain in the polyimide. With such drawbacks, this system could not be put to practical use.
To solve these problems, a method of mixing a polyimide precursor with a compound having a photosensitive group (see JP-A 54-109828, etc.), and a method of combining a polyimide precursor with a compound having a photosensitive group, thereby introducing the photosensitive group into the polyimide precursor (see JP-A 56-24343, 60-100143, etc.) have been proposed. However, since the photosensitive polyimide precursors in those methods are mainly derived from aromatic monomers having good heat resistance and good mechanical properties, but low UV transmission owing to the light absorption of the polyimide precursors themselves, the result is that the photochemical reaction in the exposed area of the polyimide film is often insufficient (i.e., there is low photosensitivity). As a result, using the above photosensitive polyimide precursors in patterning is often problematic in that the photosensitivity of the polyimide film is low and that the profile and resolution of the patterns formed is not good. With the increase in the degree of semiconductor integration in the art, the design rule in semiconductor devices has become much finer, and semiconductor devices are required to have a higher degree of resolution.
In fabricating semiconductor devices, 1:1 projectors, called mirror projectors, and reduction projectors, called steppers, are being employed, for example, in place of conventional contact/proximity projectors with parallel rays in order to image these finer circuit patterns. For steppers, one can use monochromatic light such as high-power oscillation light from ultra-high-pressure mercury lamps, or excimer laser beams. So-called g-line steppers are the most popular steppers. These steppers use a g-line visible light (having a wavelength of 436 nm) from ultra-high-pressure mercury lamps. However, in order to meet recent requirements of finer working rules, the wavelength of the light to be applied to steppers must be shortened. In this situation, i-line steppers (wavelength: 365 nm) are being used in place of g-line steppers (wavelength: 436 nm).
However, conventional photosensitive polyimide base polymers that are designed for contact/proximity projectors, mirror projectors and g-line steppers have poor i-line transparency, as mentioned above, and the i-line transmittance through the polymers is nearly 0. Therefore, patterning conventional photosensitive polyimides with i-line steppers gives poor patterns. For LOC (lead-on-chip) high-density packaging systems for fabricating semiconductor devices, thick polyimide films are desired for surface protection. The problem of poor light transmittance is even more serious in the case of thick polyimide films. Accordingly, photosensitive polyimides having high i-line transparency and capable of being patterned with i-line steppers into good patterns are highly desired. In this connection, it has been reported that introducing some substituents into the aromatic rings in the main chain of photosensitive polyimides is effective for increasing the i-line transmittance of the polyimides (see JP-A 8-337652, etc.).
The diameter of the silicon wafers used as a substrate for semiconductor devices is becoming larger. The increase in the diameter has brought about another problem in that the silicon wafers coated with a surface-protecting polyimide film are warped more than previously owing to the difference in the thermal expansion coefficient between the polyimide film and the underlying silicon wafer. In this situation, photosensitive polyimides are sought after which have thermal expansion coefficients smaller than that of conventional polyimides. In general, the thermal expansion of polyimides having a rigid rod-like molecular structure is advantageously low. However, the i-line transparency of typical polyimides having a rigid, rod-like molecular structure is low. Therefore, the ability to photopattern polyimides of this type is usually low.
In accordance with the above objects, the invention provides aromatic polyimide precursors having an increased i-line transparency which are capable of being imidized into polyimide resins with low coefficient of thermal expansion and low mechanical stress on silicon wafers, and provides a photosensitive resin composition comprising the precursor. The composition also has the advantages of good heat resistance, high photosensitivity and high resolution.
The invention also provides a photosensitive resin composition capable of being developed with an aqueous alkaline solution which is more environmentally friendly than solvent based developers.
The invention also provides a patterning method to give polyimide patterns having a good profile. The method uses the photosensitive resin composition noted above. Because of high i-line transmittance and high photosensitivity, the polyimide precursor in the composition is readily processed through i-line exposure to give high-resolution patterns. The polyimide films formed after imidization have the advantages of good heat resistance and low mechanical stress on silicon wafers.
Another advantage of the patterning method according to the invention is that an environmentally friendly aqueous alkaline solution is available for development in the method.
The invention further provides reliable electronic components having high-resolution polyimide patterns. In the electronic components, the polyimide patterns formed have a good profile and high heat resistance, and their residual stress is extremely small.
Specifically, the invention provides in its preferred embodiments the following:
(1) A photosensitive resin composition comprising an aromatic polyimide precursor, in which the light transmittance of a 10 xcexcm thick layer of precursor, at a wavelength of 365 nm is at least 1%, and a 10 xcexcm thick polyimide film made from the precursor through ring closure, when formed on a silicon wafer, results in a residual stress of no more than 25 MPa.
(2) The photosensitive resin composition of (1), wherein the light transmittance at a wavelength of 365 nm through the 10 xcexcm thick film made from the aromatic polyimide precursor is at least 10%.
(3) A photosensitive resin composition comprising an aromatic polyimide precursor, in which the aromatic polyimide precursor has repetitive, structural units of a general formula (I): 
wherein A and B each independently represent a trivalent or tetravalent aromatic group; and X and Y each independently represent an at least divalent group not conjugating with A or B.
(4) The photosensitive resin composition of (3), wherein the aromatic polyimide precursor having the structure of formula (I) has repetitive, structural units of a general formula (II): 
where A and B each independently represent a tetravalent aromatic group; X and Y each independently represent a divalent group not conjugating with A or B; Z represents an at least divalent aromatic group; R1 and R2 each independently represent a hydroxyl group or a monovalent organic group.
(5) The photosensitive resin composition of (1) or (2), wherein the aromatic polyimide precursor is that of (3) or (4).
(6) The photosensitive resin composition of (4), wherein R1 or R2 in formula (II) is a monovalent organic group having a photosensitive group.
(7) The photosensitive resin composition of (4), wherein R1 or R2 in formula (II) is a group of:
xe2x80x94Oxe2x88x92N+HR4R5xe2x80x94R6,
xe2x80x94Oxe2x80x94R6, or
xe2x80x94NHxe2x80x94R6 
where R4 and R5 each independently represent a hydrocarbon group, and R6 represents a monovalent organic group.
(8) The photosensitive resin composition of (7), wherein R6 is a group having a carbonxe2x80x94carbon unsaturated double bond.
(9) The photosensitive resin composition of (7), wherein R1 or R2 in formula (II) is a group of:
xe2x80x94Oxe2x88x92N+HR4xe2x80x94R6 
where R4 and R5 each independently represent a hydrocarbon group, and R6 represents a monovalent organic group having a carbonxe2x80x94carbon unsaturated double bond.
(10) The photosensitive resin composition of (7), wherein R1 or R2 in formula (II) is a group of:
xe2x80x94Oxe2x80x94R6 
where R6 represents a monovalent organic group.
(11) The photosensitive resin composition of any of (1) to (11), wherein the aromatic polyimide precursor is soluble in an aqueous alkaline solution.
(12) The photosensitive resin composition of (10), wherein Z in formula (II) is a group having a carboxyl group or a phenolic hydroxyl group.
(13) The photosensitive resin composition of (10), wherein Z in formula (II) is a group of a general formula (III): 
where Zxe2x80x2 represents a single bond, O, CH2, S or SO2; R11 to R18 each independently represent H, COOH, OH, an alkyl group having from 1 to 10 carbon atoms, a fluoroalkyl group having from 1 to 10 carbon atoms, a fluoroalkoxy group having from 1 to 10 carbon atoms, or a halogen atom, and wherein, optionally, at least one of R11 through R18 is COOH or OH.
(14) The photosensitive resin composition of any one of (3) to (13), wherein X and Y each independently represent a carbonyl group, an oxy group, a thio group, a sulfinyl group, a sulfonyl group, an optionally-substituted alkylene group having from 1 to 5 carbon atoms, an optionally-substituted imino group, an optionally-substituted silylene group, or a combination of any of these groups.
(15) The photosensitive resin composition of (14), wherein X and Y each independently represent an oxy group, a thio group, a sulfonyl group, an optionally-substituted methylene group, or an optionally-substituted silylene group.
(16) The photosensitive resin composition of (15) wherein X is an optionally-substituted methylene group, and Y is an oxy group.
(17) The photosensitive resin composition of any one of (3) to (16), wherein A and B are both benzene rings.
(18) The photosensitive resin composition of any one of (1) to (17), which further contains a photopolymerization initiator and which has a negative-type photosensitive characteristic.
(19) The photosensitive resin composition of any one of (1) to (17), which further contains a compound capable of generating an acid in light and which has a positive-type photosensitive characteristic.
(20) A method for forming patterns, which comprises applying the photosensitive resin composition of any one of (1) to (19) onto a substrate and drying, exposing the composition, developing the composition, and heating the composition to form a pattern layer.
(21) The patterning method of (20), wherein i-line radiation is used as the light source in the exposing step.
(22) The patterning method of (20) or (21), wherein the substrate is a silicon wafer having a diameter of at least 12 inches.
(23) Electronic components having a patterned layer according to the method of any one of (20) to (22).
(24) Electronic components of (23) for semiconductor devices, wherein the patterned layer is for a surface-protecting film or an interlayer insulating film.
Further objects, features and advantages of the present invention will become apparent from the Detailed Description of preferred embodiments, which follows, when considered together with the attached drawings.